In recent years, the tendency of crude oil becoming heavier and worse is growing more significant day after day world wide. Meanwhile, the demand for middle distillates is continuously increasing. These lead to a rapid development of the hydrocracking technique for the purpose of the bettering ad the lightening of heavy distillate, while the hydrocracking catalyst is one of the most important and critical factors of the hydrocracking technique.
Hydrocracking catalyst is a bifunctional catalyst which possesses at the same time the cracking activity and hydrogenation activity. In other words, hydrocracking catalyst contains at the same time acidic component and hydrogenation component. The acidic component is mainly the refractory inorganic oxide and/or various zeolites, and the hydrogenation component is generally selected from the oxides of the metals selected from Group VIB or Group VIII of the Periodic Table. In order to obtain different hydrocracking products, it is required to regulate the cracking activity and the hydrogenation activity of the catalyst. Generally speaking, the hydrocracking catalyst ought to possess good performances of desulfurization, denitrogenation and hydrogenation for aromatic compounds. The catalyst used for producing middle distillates should further possess high selectivity to middle distillates and nitrogen resistant stability. In general, the selectivity and the nitrogen resistant stability of the hydrocracking catalyst re raised by means of regulating the acidity of the catalyst.
Chinese patent application No. CN 90102649.4 discloses a hydrogenation catalyst. Said catalyst contains 0.5-5.0 wt % fluorine, 2.5-6.0 wt % nickel oxide, 10-32 wt % tungsten oxide, mordenite having silica to alumina ratio of 9.5-10.5 or Y-type zeolite having silica to alumina a ratio of 4.5-5.5, as well as alumina. Said alumina is prepared by calcining hydrated alumina which is obtained by hydrolysis of alkoxy aluminum. The boehmite content of the alumina is greater than 65 wt %. Although said catalyst possesses high hydrogenation activity and cracking activity, it is merely used for producing raw materials for steam cracking process, while not suitable for producing middle distillates, and the cost of its carrier is rather high.
U.S. Pat. No. 4,894,142 discloses a mid-barrel or distillate hydrocracking process, wherein a catalyst containing metal components selected from Group VIB and Group VIII of the Periodic Table, refractory inorganic oxide and a Y-type zeolite is used. Said Y-type zeolite has a unit cell size of 24.20-24.40 Angstroms, the ion exchange capacity greater than 0.07 and tie acidity strength value determined by NH.sub.3 -TPD less than 2.00.
In addition, U.S. Pat. No. 4,419,271, U.S. Pat. No. 4,401,556 as well as U.S. Pat. No. 4,517,073 also disclose respectively hydrocarbon conversion catalysts or hydrocracking catalysts, characterized in that the Y-type zeolite possessing certain specific silica to alumina ratio and modified by various methods is used therein.
One of the objects of the present invention is to provide a hydrocracking catalyst which is suitable fo producing middle distillates and has high nitrogen resistant stability.
The other object of the present invention is to provide a process for the preparation of such hydrocracking catalyst.
As is indicated above, the selectivity to middle distillates and the nitrogen resistant stability of the hydrocracking catalyst are enhanced usually by means of regulating the acidity of the catalyst In accordance with the prior art the acidity of the catalyst is regulated mainly by means of changing the species, the content, or the properties of the zeolite used for the carrier of the catalyst. The inventors of the present invention found that a hydrocracking catalyst possessing high selectivity to middle distillates and excellent nitrogen resistant stability can be obtained by means of selecting the alumina and the zeolite of adequate acidities for the preparation of the carrier for the hydrocracking catalyst, i.e., the alumina possessing specific acidity strength value is made to be matched with the zeolite of a specific acidity strength value.